On board a rotorcraft, an autopilot (“AP”) transmits command signals or data to electric actuators that are connected in series in the flight controls. A compensation actuator is also fitted to the flight controls, and in particular in the pitch control system and in the roll control system in order to move the anchor point (or “midpoint”) of the manual control (“cyclic stick” in particular) that is made available to a human pilot.
Such a compensation actuator, also referred to as a “trim” actuator or as a “parallel” actuator is also controlled by signals delivered by a computer of the AP. Together these actuators serve to control variation in the position of an aerodynamic surface (or member), such as a rotor blade or an aileron, as a function of a position setpoint signal for that surface, while also enabling the human pilot to keep control over the flight controls.
The invention applies in particular to a compensation actuator including an electric motor driving an outlet shaft via a speed-reducing gearbox.
Patent FR-A-2 718 102 describes such a compensation actuator in which the gearbox is reversible, in which the motor is autosynchronous, and that includes a sensor responsive to the angular position of the outlet shaft, a sensor responsive to the angular position of the motor shaft, and an electronic control device that receives signals from said sensors and that applies electric currents to the motor suitable for obtaining a torque that corresponds to a force opposing the movement of a flight control lever.
As stated in that patent, it is also known to make a “trim” actuator in which the transmission comprises, in succession and coupled in this order between the motor and the outlet shaft: an irreversible gearbox; a clutch; a viscous damper; a second gearbox; a spring box; and a safety device.